This invention relates to light-emitting semiconductor devices, wherein carriers are injected across a junction into a region so that the light is emitted when the carriers recombine in the region, and relates to semiconductor devices having a schottky barrier with high breakdown voltage.
In a semiconductor device having a p-n junction, when a electric field is applied to allow a forward current to flow, minority carriers are injected across the junction, and when the minority carriers recombine with the majority carriers, as infrared or visible light is emitted. Such light-emitting phenomenon is utilized in a known light-emitting diode. Furthermore, by confining the emitted light as well as the carriers in a small limited region of a specified size, a light of a constant wavelength and of a coherent phase is amplified and can be taken outside. This is called the light-amplification by stimulated emission of radiation, or laser.
Recently, semiconductor lasers have made a remarkable progress, and especially, with the invention of the known doublehetero structure of semiconductor laser, the semiconductor lasers have entered into practical uses.
In the doublehetero structure, the emitted light as well as the injected carriers are confined in a thin layer or region which is substantially vertical to the direction of the current in the laser. By means of such confinement, the efficient lasing i.e., the laser oscillation, becomes realized.
As a typical example of the doublehetero structure laser, a laser constituted by sandwiching GaAs(gallium arsenic) layer, which is a semiconductor of III-V compound, with two GaAlAs (aluminum-gallium arsenic) layers is well known through, for instance, United States Patent specification No. 3,691,476 by Hayashi, Applied Physics Letters Aug. 1, 1970 vol. 17, No. 3, or Applied Physics Letters April 15, 1970, vol. 16, No. 8.
The abovementioned GaAlAs is, in exactness, Ga.sub.1-x Al.sub.x As, which is a mixed crystal obtained by replacing a part of Ga of GaAs with Al, wherein the replaced ratio is indicated by .times. (0&lt;.times..ltoreq.1). Such layers of GaAs and GaAlAs are formed with known liquidphase epitaxial growths.
Such semiconductor laser is very small in size, its active region where the current is concentrated being particularly small, and the current density, hence the heat loss, in the region is very great, and therefore, it is desirable to radiate this heat away. It is all the more important to effectively radiate the heat, since the more effective the radiation is, the higher the continuous wave power becomes possible.
It is also important in the semiconductor laser to effectively concentrate a current in the active region, hence to decrease threshold current, as well as to obtain single mode lasing in the laser. These are for obtaining laser light of high quality for use as a medium of information processing or in communication.
For clear understanding of the present invention, a typical doublehetero structure semiconductor laser of prior art is elucidated referring to FIG. 1 and FIG. 2.
The known laser of FIG. 1 is made by sequential liquid-phase epitaxial growths on an n-type GaAs substrate 1, thereby sequentially forming
a first layer 2 of n-type Ga.sub.1-x Al.sub.x As,
a second layer 3 of p-type GaAs,
a third layer 4 of p-type Ga.sub.1-x Al.sub.x As,
said first to third layers forming a doublehetero structure, and
a fourth layer 5 of p.sup.+ -type GaAs, and
further forming metal electrodes 6 and 7 for ohmic contactings to the bottom face of the n-type GaAs layer 1 and to the top face of the p.sup.+ -type GaAs layer 5, respectively.
The abovementioned laser of FIG. 1 works as a doublehetero structure laser wherein the p-type GaAs layer 3 is the active region sandwiched by n-type Ga.sub.1-x Al.sub.x As layer 2 and p-type Ga.sub.1-x Al.sub.x As layer 4, and both the carriers and the light are confined in the active region 3.
In such laser, the range of x in said Ga.sub.1-x Al.sub.x As is generally selected to be 0.25&lt;.times.&lt;1.0. The lower end of the range is defined by the following reasons:
Band gap and refractive index of the substance Ga.sub.1-x Al.sub.x As is dependent on the value x. And, for the value of 0.25&lt;.times. the band gap of the Ga.sub.1-x Al.sub.x As becomes large enough to confine the injected carriers in the sandwiched GaAs layer, and also the refractive index of the Ga.sub.1-x Al.sub.x As becomes small enough to confine light in the active region 3 of GaAs.
In the laser of FIG. 1, the metal electrodes 6 and 7 are formed on all the bottom and top faces of the wafer, respectively, and therefore, the lasing output is large, but has a shortcoming that the threshold current becomes large and moreover the lasing mode becomes multiple.
In order to improve the abovementioned shortcoming, the known stripe-type semiconductor laser has been developed.
FIG. 2 shows a typical example of such known stripe-type laser, wherein a silicon-oxide insulating film 8 is provided on the fourth layer 5 of p-type GaAs and a stripe-shaped narrow window or groove 10 is formed in said insulating film 8 so as to expose a part of the fourth layer 5, and, further, a metal electrode 71, for instance, of gold, is provided on the wafer, so that the metal electrode 71 covers the insulating film 8 and the groove 10. Other parts are constituted similarly to the prior art of FIG. 1.
In the stripe-type laser of FIG. 2, the current is fed through a stripe-shaped part of the metal electrode, the part contacting the fourth layer 5 at the bottom of the groove 10. Accordingly, the current flows in through the narrow stripe part of the electrode and hence, the carrier is concentrated into the narrow region right under the groove. Therefore, an account of the abovementioned concentration, the threshold current is considerably decreased, and also the lasing mode can be made simple.
In the known stripe-type laser of FIG. 2, the silicon-oxide insulating film 8 which has poor heat conductivity is provided between the fourth layer 5 and the metal electrode 71. Therefore, the radiation of heat from the active region through the fourth layer 5, insulating film 8 and metal electrode 71 to upper outside space is very poor, and the temperature of the active region is raised. On account of the abovementioned poor heat radiation, it is difficult to obtain high lasing output at continuous wave operation. Furthermore, in the conventional laser of FIG. 2, thermal expansion coefficient of the forth layer 5 of GaAs and that of the insulating film 8 prominently differ from each other, and therefore, a considerable strain or defect is produced at the interface inbetween, and such strain or defect causes considerable adverse defect further in the active region, i.e., second layer 3. As a result of the defect in the active region 3, the lasing performance, especially of differential quantum efficiency is lowered and also the life of the laser device is shorten. In order to reduce such adverse effects of the defects to the active region, it is necessary to increase thickness of the fourth layer 5 of p-type GaAs. However, the increase of thickness of the fourth layer 5 causes dispersion of injection current in the widthwise direction of the groove 81, and accordingly causes the threshold current to increase, hence, poor heat radiation. Undesirable temperature rise due to the poor heat radiation disables the lasing of short wavelength light.